It can be alarming to install a performance plugin only to see your GTmetrix Grade drop from an A to a B, or your Largest Contentful Paint (LCP) increase.

Don't panic! In 99% of cases, the plugin has not slowed down your site. Instead, you are likely seeing a temporary testing anomaly caused by how GTmetrix interacts with caches and CDNs.

If your scores dropped right after installation, check these three common culprits:

1. The "Unprimed Cache" Effect (Run the test 3 times!)

When you run a GTmetrix test immediately after installing the plugin or clearing your Joomla cache, the server has to work overtime. It is generating new WebP images and building the cache on the fly exactly while GTmetrix is measuring the load time.

  • The Fix: Never trust the first test. Run the GTmetrix test, wait 2 minutes, run it a second time, and then run it a third time. The third test shows the true, cached speed that your actual visitors will experience.

2. CDN Synchronization Delays (Cloudflare, etc.)

If you use a Content Delivery Network (CDN) like Cloudflare, it takes time for them to fetch the newly optimized images from your Joomla server and distribute them to their global edge nodes.

  • The Fix: If you just enabled JR Image Optimizer, purge your CDN cache entirely. Run a few tests to force the CDN to pull the new lightweight images, and then wait 15–30 minutes before trusting the GTmetrix report.

3. GTmetrix Testing Server Location

Are your website and your target audience located in Europe, but your free GTmetrix account is testing from Vancouver, Canada?

  • The Fix: Always ensure your testing location matches where your server (or primary audience) is located. Distance adds latency, which artificially inflates your LCP metric. If you cannot change the test location, the data will always be slightly skewed.

 

Still struggling with massive image payloads in Joomla?

If you are spending hours manually compressing images or fighting with PageSpeed Insights, you might need a native solution.

We built JR Image Optimizer specifically for Joomla. It automatically converts your images to next-gen WebP formats, serves them responsively, and fixes Layout Shifts (CLS) without you having to lift a finger. Click here to see how it can transform your Joomla site's performance.

🔍 Still seeing issues? Check the Waterfall!

Don't just look at the letter grade; look at the Waterfall chart in GTmetrix. Filter by "Images."

  • Check the file sizes. Are they smaller than before?
  • Check the file types. Are they loading as .webp? If the images are smaller and loading in next-gen formats, the plugin is working perfectly. Your LCP delay might actually be caused by a third-party script, an unoptimized font, or a slow server response time (TTFB) happening just before the image loads.